1. Field of the Invention
The present invention generally relates to a display panel and fabrication method thereof. More particularly, the present invention relates to an active matrix organic electro-luminescent display (AMOELD) panel and manufacturing method thereof.
2. Description of the Related Art
Telecommunication has become an important activity in our society thanks to the development of various communication and display products. Because display panel is an indispensable man-machine interface in communication, its development is particularly important. Among the display panels, organic electro-luminescent display (OELD) has the greatest potential for further development because it has many advantages including a self-illuminating screen, wide viewing angle, a low power rating, a wide temperature operating range, a high response speed and full coloration. Moreover, the OELD panel is easy to fabricate and has a low cost of production.
Organic electro-luminescent display (OELD) panel is a display that utilizes the self-illuminating property of an organic light-emitting material to illuminate an image. According to the molecular weight of the organic light-emitting material, the OELD panel can be classified into small molecule organic electro-luminescent display (SM-OELD) and polymer electro-luminescent display (PELD). The light-emitting structure of both types of OELD comprises a pair of electrodes and an organic material layer sandwiched between the two. When a DC voltage is applied to the electrodes, holes are injected from the anode into the organic light-emitting material layer while electrons are injected from the cathode into the organic light-emitting material layer. Due to the potential produced by an external electric field, hole and electron carriers moving inside the organic light-emitting material layer may collide and recombine with each other. A portion of the energy released by the recombination of electron-hole pairs may excite the organic light-emitting molecules into an excited state. When an excited molecule releases its energy and falls back to a ground state, a definite portion of the energy is released as photons. Hence, the organic electro-luminescent display (OELD) panel will emit light on activation.
In recent years, an active matrix organic light-emitting display (AMOLED) is being actively developed. The AMOLED panel is fabricated by forming an anode layer, an organic light-emitting material layer and a cathode layer sequentially over a substrate having a plurality of thin film transistors thereon. It should be noted that the cathode layer is formed over the organic light-emitting material layer globally. With this design, the cathode provides a common operating voltage. However, because the light-emitting efficiency, preferred operating voltage and brightness attenuation are not identical for different organic material, the design of the cathode layer and the method of operating the cathode can hardly meet the demand for full coloration in a display device.
In the Japanese Patent No. 10-3199910, the current differential between a scan electrode and a common electrode is sampled and compared with a reference signal to determine the input current, thereby adjusting the brightness level. However, this type of design has a rather complicated circuit. In another Japanese Patent No. 11-073158, a different number of light-emitting devices are used in different color pixel areas to combat any variation in light emission and hence the brightness level. Although this method is capable of bringing the brightness level of different colors closer together, the changes in pixel areas often leads to a difference in picture contrast and a lowering of resolution and complicates the design of the driving circuits. In yet another Japanese Patent No. 11-073159, a transformation table relating the display data with the properties of various materials is set up. According to the input signal, a voltage corresponding to the material is generated to bring the brightness level closer together. However, one major defect is that slight modification of the material properties may invalidate the transformation table.